The present invention relates to a method for balancing background conductivity in the analysis of anions in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the ion chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,956,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin bed. A different form of suppressor column is described in U.S. Pat. No. 4,474,664, in which a charged membrane in the form of a fiber or sheet is used in place of the resin bed. In sheet form, the sample and eluent are passed on one side of the sheet with a flowing regenerant on the other side of the sheet. The sheet comprises an ion exchange membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
In a typical commercial suppressor, a small amount of electrolyte passes in the effluent to the ion conductivity detector as background. Of course, such background conductivity is relatively low in comparison to the level of conductivity which would pass to the detector without use of the suppressor. However, the background conductivity can cause problems for an eluent of the gradient type wherein the concentration of electrolyte is progressively increased during the run. In such instances, the baseline of a chromatogram reflecting such background correspondingly increases adding difficulty in analyzing the chromatographic peaks. Sometimes, the background causes the baseline to go off-scale. It would be desirable to provide a way to maintain relatively constant baseline background conductivity during a gradient eluent run.
It is further known that boric acid, while a weak acid by itself, in the presence of polyhydroxy compounds such as glycerol or mannitol, acts as a much stronger acid. The boric acid and polyhydroxy compounds form a highly ionized complex. A binary composite solution of these type of compounds has been suggested for use as the eluent for analysis of anions in Miyanaga et al. U.S. Pat. No. 4,500,431.